The invention relates to a rotary joint with two housing parts rotatable in relation to each other, which are connected by a flexible flat conductor that extends in spires within an interstice between the housing parts and is attached at its end to a contact element, which connects a strand of the flat conductor with a circular conductor and which is arranged in a holder in a housing part.
The problem of suitable current or signal transmission between parts of an apparatus that can be rotated in relation to each other within a limited angular range is frequently encountered. For example, a permanent, reliable connection to supply power to deploy the airbag in a motor vehicle steering wheel is of decisive importance. Since sliding contacts are unsuitable for this purpose due to wear and fluctuating contact resistance, rotary joints with flat conductors have been developed to ensure safe connection.
The flat conductor (hereinafter referred to as FFC) is arranged in the interstice between two rotary joint housing parts rotatable in relation to each other and is attached at its ends to the two housing parts. It is normally made of one or several copper strips spaced at a distance from each other and embedded between two plastic strips. In the simplest case, the flat conductor extends helically like a clock spring. On relative rotation of the housing parts, the diameter and the spire number changes as a function of the direction of rotation. As an alternative, the spires of the flat conductor extend along the inner and outer radial walls of the interstice in opposite directions of rotation, whereby there is a reversing point between the two conductor segments. In this case, upon relative rotation of the housing parts of the joint, the conductor is unwound from one wall and taken up by the other. It is also well known to arrange guiding or elastic elements within the interstice and to make the flat conductor from an elastic material to ensure its safe guidance upon rotation of the housing parts and to prevent damage.
The exterior contacting of the rotary joint is generally effected by wires or stranded conductors with an approximately circular cross-section. Consequently, contact elements have to be provided, which electrically connect the circular conductors with the strands of the flat conductor. Such a contact element is described, for example, in DE 196 04 797 A1. Its design is complex, however, which is a disadvantage particularly if production lots are small, since forming dies are required to produce the contact element. Furthermore, the described contact element has relatively large dimensions and is arranged in the radial outer wall of the rotary joint. Since many functional elements are disposed on the steering wheel of a motor vehicle, the corresponding mounting space is frequently not available.